The present invention relates to a method and article for the electrical contacting of thermoelectric semiconductor elements.
Thermoelectric materials have the ability to convert heat directly to electricity without conventional rotating machinery. Thermoelectric generators are therefore highly desirable power sources for portable and remote applications. This is particularly the case where the power and life requirements of the generator are such as to make batteries, solar cells, or other electrical generators less attractive due to higher weight-to-power ratios, fuel requirements, noise, or other undesirable characteristics under severe environmental conditions. Thermoelectric materials are well known to the art and include such materials as germanium-silicon, zinc-antimony, copper-silver-selenium, bismuth telluride, lead telluride, germanium-bismuth telluride, tin telluride, lead-tin telluride, and Chromel-constantin.
A thermoelectric converter assembly generally comprises an array of thermoelectric materials, alternately doped with N-type and P-type dopants with electrical contacts joined thereto. One side of the element is connected to a hot junction or shoe in communication with a heat source, and the other side to a cold junction or shoe in communication with a heat sink such as an environmental radiator. The temperature differential impressed across the thermoelectric material serves to generate a voltage, in accordance with the Seebeck effect. Individual thermoelectric elements are connected by electrical leads, such as of copper, which are ordinarily brazed to the shoes.
The bonding of thermoelectric materials to electrical contacts imposes a number of severe materials constraints. The current-carrying ability of a thermoelectric material depends, as is known, upon the concentration and the purity of the thermoelectric material itself and of the dopants added thereto. N-type PbTe is made, for example, with PbI.sub.2 as the dopant, P-type PbTe is doped with sodium, and P-type PbSnTe is doped with manganese. Trace amounts of certain other metals, such as copper, nickel, or chromium upset the necessary balance in the thermoelectric material, and thus by degrading current-carrying ability or affecting polarity, are said to be "poisonous". Therefore, such metals and their alloys cannot be used as the directly-facing electrical contacts for thermoelectric materials, although they may be good current conductors. On the other hand, there are other materials, such as pure iron, which do not poison thermoelectric semiconductors, and have in the past been used as contacting shoes to the copper electrical leads. Shoes of iron, however, are found to have certain drawbacks when utilized in high-temperature modules and in those which undergo frequent thermal cycling. These drawbacks derive principally from the fact that many thermoelectric materials, and in particular those containing tellurium, have thermal expansion coefficients which are far greater than that of iron. As a result of such thermal mismatch, the fragile and brittle thermoelectric materials are subject to fracture and other damage. This is because iron and telluride will not expand at the same rate at temperature, principally in the unrestrained radial direction, and will separate.
The principal object of the present invention, therefore, is to provide an improved method of bonding a thermoelectric material to an electrical contact.
Another object is to provide a method of bonding a tellurium-containing thermoelectric semiconductor to an electrical shoe in such a manner as to avoid poisoning of the thermoelectric material or thermal mismatch, while obtaining an efficient electrical contact.
Another object is to provide a bonded electrical contact between a thermoelectric material and an electrical conductor which is compatible thermally and electrically and which does not introduce poisons into the thermoelectric material.
The single drawing is a schematic representation of the bonded electrical contact for thermoelectric semiconductors provided by the present invention.